Vitamin D is required for the regulation of the minerals calcium and phosphorus found in the body. It also plays an important role in maintaining proper bone structure. It is important in non-skeletal tissues, and its deficiency is closely associated with increased risk of cancers, infections, autoimmune diseases, cardiovascular diseases, and diabetes mellitus. Vitamin D is mainly produced in the skin when directly exposed to sunlight, or obtained from the diet. Vitamin D receptors are expressed on immune cells (B cells, T cells and antigen presenting cells) and these immunologic cells are all capable of synthesizing the active vitamin D metabolite. Vitamin D can also modulate the innate and adaptive immune responses. Deficiency in vitamin D is associated with increased autoimmunity as well as an increased susceptibility to infection.